Flexographic printing plates are known for printing images on surfaces that are either rough (for example, corrugated) or smooth, such as packaging materials, plastic films, wallpaper, and fabrics. The process has mainly been used in the packaging industry where the plates should be sufficiently flexible and the contact sufficiently gentle to print on uneven substrates such as corrugated cardboard as well as flexible materials such as polypropylene film. For flexographic printing, a flexible plate with a relief image is usually wrapped around a cylinder and its relief image is inked up and the ink is then transferred to a suitable printable medium. In order to accommodate the various types of printing media, the flexographic plates should have a rubbery or elastomeric nature whose precise properties can be adjusted for each particular printable medium.
The flexographic printing plate may be prepared by exposing the UV sensitive polymer layer of the plate through a mask in the form of a negative film. The process involves a number of other stages such as a back UV exposure before imaging, a solvent or thermal development stage and heating and further UV exposures. For the purposes of the present invention as described below, flexographic plate imaging using a negative film through which the plate is UV exposed and further treated will be termed the “conventional process.” This will distinguish it from digital imaging that may include LAMS (laser ablated mask) and direct engraving.
In addition, radiation-sensitive elements having a laser-ablatable element integral to the surface are also known in the art. A relief image can be produced in such elements without the use of a digital negative image or other imaged element or masking device. Also, films with a laser-ablatable mask layer can be formed by first imagewise exposing the film with laser radiation (generally an infrared radiation laser under computer control) to selectively remove the mask layer in the exposed areas.
The masking film is then placed in contact with a radiation-sensitive element and subjected to overall exposure with actinic radiation (for example, UV radiation) to cure the radiation sensitive element in the unmasked areas and thus form a negative image of the mask in the element. The film containing the mask layer and the imaged radiation-sensitive element (such as an imaged printing plate precursor) are then subjected to solvent development. The unexposed printing plate areas and the mask layer are completely developed off, and after drying, the resulting imaged element is useful, for example as a flexographic printing plate.
A simpler way of making a flexographic printing plate is by direct engraving using laser beam ablation, thereby eliminating all need for washing or drying the plate or multiple types of exposure.
Once a plate has been imaged by any of the above methods, there is very little that can be done if an error has crept in to the process or the plate that has been prepared for printing needs correction in any way and it is then necessary to prepare another plate by the long processes of exposure and development as described above.
Conventionally imaged flexographic plates are generally imaged flat by placing them in a vacuum frame with the negative film in contact. The finished elastomeric plate will then have to be wrapped around the cylinder of the printing press and this results in distortion of the image. Some distortion factor formulae exist for modifying the exposure of the negative to correct for distortion, but such methods are inexact. The process of correction is known as “dispro.”
Similarly, where a LAMS plate is imaged on a drum using a laser, ablation occurs and the flexographic plate is then UV-exposed frequently in the flat form and the finished plate is eventually put on the printing press cylinder of a different diameter to the exposure system some image distortion can easily be introduced. In order to avoid this type of distortion, it is possible to image and print on a sleeve that both fits into the imaging system and the printing system. This way the flexographic element is not removed and repositioned during the entire process from before imaging to after printing. This method does not lend itself to conventional exposure where a vacuum would have to be exerted in the round. But it does lend itself to both LAMS and direct engraving.
One method for using sleeves is known as plate-on-sleeve. The precursor plate is bonded to an inert sleeve shell and it can then be imaged and further processed without removal from the shell. The most advantageous method in many respects is where the customer receives the sleeve coated with a seamless flexographic plate precursor that can then be imaged and treated and used for printing. Such a method commends itself as several sleeves can have color separated images accurately positioned for preparation of color prints where the colors must be printed accurately one on top of another. This saves the printer considerable time and effort in setting up the print cylinders to produce the same accurate register effect. Also, seamless sleeves can be used to produce an endless continuous pattern. Even where there is a repeat pattern, wastage of material corresponding to the unprintable seam area of the plate can be saved by using a seamless sleeve. Seamless sleeve flexographic plates can be used for printing at faster printing speeds than plates or plates-on-sleeve because at fast speeds flexographic plates that are bonded to the cylinders so that they can subsequently be removed tend to lift off at the edges when used too fast. A disadvantage of seamless sleeves is that they are generally more expensive than other types of flexographic printing plate precursors and the storage of sleeves requires more space than flat printing plates.
In recent years, the quality of flexographic prints (impressions) has improved markedly, but a significant obstacle to flexography gaining a greater share of the print market is the cost of the flexographic printing precursor, whether it is a printing plate precursor or printing sleeve precursor. The precursor may be purchased by either a trade shop that prepares the printable flexographic plate or sleeve for a printer to use or it may be purchased directly by the printer. At this point the plate is imaged by a computer-to-plate (CTP) device. In the case where an error is detected in the content of the imaged plate, then it becomes unusable, the trade shop or printer (or the user) has to absorb the cost (loss). In cases where the imaged plate could adequately be repaired for use it would be a considerable advantage.